Oracle SQL & PL/SQL

1. What is denormalization? A. Removing tables B. Combining tables to improve performance C. Deleting duplicate data D. Creating indexes only Ans : B 2. What is the main advantage of denormalization? A. Less storage B. Better normalization C. No redundancy D. Faster query performance ANS : D 3. What is a disadvantage of denormalization? A. Increased redundancy B. Better reporting C. Reduced JOINs D. Faster queries Ans : A 4. Which systems commonly use denormalization? A. Banking transaction systems B. Reporting and dashboard systems C. Authentication systems D. Compiler systems Ans : B 5. Which statement is TRUE? A. Normalization improves performance only B. Denormalization reduces redundancy C. Normalization reduces redundancy D. Denormalization removes duplicate data Ans : C 6. What is the purpose of constraints in SQL? A. Increase redundancy B. Maintain data integrity C. Speed up internet D. Delete tables Ans : B 7. Which constraint prevents NULL values? A. DEFAULT B. UNIQUE C. NOT ...

 ✅ SQL Date & Time Functions ✅ 1. Why Date Functions Matter?   Almost every real-world database contains dates 👇   ✔️ Orders   ✔️ Employee joining dates   ✔️ Transactions   ✔️ Login activity   SQL date functions help analyze time-based data 💯   ⚡ 2. Common Date Functions   Function : Purpose   NOW() : Current date & time   CURDATE() : Current date   CURTIME() : Current time   YEAR() : Extract year   MONTH() : Extract month   DAY() : Extract day   DATEDIFF() : Difference between dates   DATE_FORMAT() : Format dates     🔥 3. NOW(), CURDATE(), CURTIME() SELECT NOW(); Output : ✔️ Current date + time SELECT CURDATE(); Output : ✔️ Current date only SELECT CURTIME(); Output : ✔️ Current time only   🔥 4. YEAR(), MONTH(), DAY() SELECT YEAR(joining_date) FROM employees; SELECT MONTH(joining_da...

  ✅ SQL String Functions    🧠 1. What are String Functions?   String functions are used to   👉 manipulate text data   👉 clean messy data   👉 format outputs   Used heavily in:   ✔️ Data Analytics   ✔️ Reporting   ✔️ ETL processes   ⚡ 2. Common String Functions   Function  : Purpose   UPPER() : Convert to uppercase   LOWER() : Convert to lowercase   LENGTH() : Count characters   CONCAT() : Join strings   SUBSTRING() : Extract part of string   TRIM() : Remove spaces   REPLACE() : Replace text   🔥 3. UPPER() & LOWER() SELECT UPPER(name) AS upper_name FROM employees; SELECT LOWER(name) AS lower_name FROM employees; 🔥 4. LENGTH()   👉 Count number of characters SELECT name, LENGTH(name) AS total_chars FROM employees; 🔥 5. CONCAT()   👉 Combine strings SELECT CON...

  🧠 1. What is NULL in SQL?   NULL means  :  👉 missing value   👉 unknown value 👉 no data available   ⚠️ NULL is NOT:   ❌ 0   ❌ Empty string   ⚡ 2. Problems with NULL Values   NULL can affect:   ❌ Calculations   ❌ Comparisons ❌ Reports   Example 👇 SELECT salary + bonus FROM employees; If bonus is NULL → result becomes NULL ❌   ======================================================== 🔥 3. COALESCE Function   👉 Replaces NULL with another value   ✅ Syntax COALESCE(column, value) ⚡ 4. Basic Example SELECT name,        COALESCE(bonus, 0) AS bonus FROM employees; ✔️ If bonus is NULL → shows 0 instead   ⚡ 5. Multiple Values with COALESCE SELECT name,        COALESCE(phone, email, 'No Contact') AS contact FROM employees; ✔️ Returns first non-NULL value   🔥 6. IS NULL & IS NOT NULL...

  ✅ CASE Statement in SQL   🧠 1. What is CASE Statement? CASE is used for conditional logic in SQL It's like if-else in programming Helps categorize data Create custom labels Build smart reports   ⚡ 2. Basic Syntax SELECT column_name,        CASE            WHEN condition THEN result            ELSE result        END AS alias_name FROM table_name; 📊 Example Table   name : Ajit, Neha, Sujit   salary : 70000, 40000, 55000   🔥 3. Basic CASE Example   👉 Categorize employees by salary SELECT name, salary,        CASE            WHEN salary >= 60000 THEN 'High Salary'            WHEN salary >= 50000 THEN 'Medium Salary'            ELSE 'Low Salary'        END AS salary_category FROM employees;...

 ✅ UNION & UNION ALL in SQL   🧠 1. What is UNION?   UNION is used to combine results from multiple SELECT queries   "Merge data from two tables into one result”   ⚡ 2. Rules for UNION    Same number of columns    Same datatype/order of columns   📊 Example Tables   👨‍💼 employees_2025 Name   - Ajit - Neha   👨‍💼 employees_2026   name   - Sujit  - Neha   🔥 3. UNION Example SELECT name FROM employees_2025 UNION SELECT name FROM employees_2025; ✔️ Removes duplicates automatically   ✅ Result   name   - Ajit - Neha   - Sujit   ⚡ 4. UNION ALL SELECT name FROM employees_2025 UNION ALL SELECT name FROM employees_2026; Keeps duplicates   Faster than UNION   ✅ Result   name   - Ajit - Neha   - Sujit   - Neha   🔥 5. UNION v...